The advent of personal computers spawned a whole new industry, that of desk top publishing. In desk top publishing applications it is possible to compose an advertisement, a book, a magazine, on the computer screen, print it on a suitable printer and have what is known in the publishing trade as camera ready copy. If the work involves color graphics then obviously a color printer is required. If the work involves no color graphics, i.e., black and white graphics or text only, then the work can be printed out as before or it can be electronically transferred to wherever it is required.
In the case of desk top publishing with color graphics it is very important that the color or colors of the graphic created on the screen by the operator match very closely the color(s) generated by the color printer. If there is any mismatch then the color master produced as hard copy will be different from what the creator intended. This is necessary because in many cases the creator may not have the opportunity of testing the color match by a test print. The creator's work may be going straight into a magazine, book or whatever, and will only be seen in hardcopy when the final product is actually printed. So it is very important to the desk top publishing professional that if they call up colors magenta, green and white on the screen, the electronic descriptions of these exact colors are transferred to the printer. Since the computer doesn't know what green or white looks like it can only go on the color information contained in its color lookup table in pseudo-color systems or, in true color systems, the color information contained in the pixel registers. This information is loaded up by the operating system on power on. It can also be changed at any time by the application program which is running.
If there are any errors in the video display circuitry which might cause one color output to be greater or less than the others or just different from that programmed, then the resultant color on the screen will not be the color the computer "thinks" it is generating and there is no means of reconciling the screen color and the printer color. Errors between red, blue and green guns can occur due to many factors both internal to the video DAC and external. Internal causes would include the silicon processing itself causing mismatches between the three channels. External causes would include the mismatching of the load resistors, cabling, or monitor.
Up to recently the simplest method of ensuring that computer colors were correctly calibrated was to have the computer display a color bar on the screen. The user would then physically hold up a color card to the screen and compare each of the displayed colors to the card colors and check that they were the same. If some difference is noted then the color adjust controls on the monitor would be manually adjusted to achieve correct colors. Systems are now becoming available which do the comparison electronically. This essentially entails holding up a small camera against the screen while a diagnostic program is running.
Whatever the mechanism employed, it is presently necessary to manually adjust a resistor or other component to effect a change in the red, green or blue currents I.sub.OR, I.sub.OG, I.sub.OB which effect the color portrayed on the display. In some cases the adjustment can only be done conjointly for all three colors red, blue and green. In other cases they can be adjusted independently relative to one another. While adjustments could be made forcefully by directly changing output or input currents or even modifying biases or references, such approaches involve extra components external to the basic RAM-DAC systems which may require different fabrication processes unsuited to manufacture on the same chip with the DACs. This adds to the cost and complexity of the ultimate device and may even affect its precision and reliability.